1. Field of the Invention
The present invention relates to a display unit, and more particularly, it relates to a display unit employing an organic electroluminescence element.
2. Description of the Prior Art
Electroluminescence (EL) elements include an organic EL element employing an inorganic compound thin film of selenium or zinc as a luminescent material and an organic EL element employing an organic compound as a luminescent material.
The organic EL element has the following excellent characteristics:
(1) High luminous efficiency.
(2) Low driving voltage.
(3) Capable of displaying various colors such as green, red, blue and yellow by selecting the luminescent material.
(4) Capable of making sharp display with no backlight due to spontaneous luminescence.
(5) Wide viewing angle due to surface luminescence
(6) Thin and lightweight.
(7) Capable of preparing a substrate from a soft material such as a plastic film due to a low maximum temperature in the fabrication process.
Due to the aforementioned excellent characteristics, the organic EL element is recently watched with interest as a display unit substituting for a cathode ray tube (CRT) or a liquid crystal display (LCD).
A dot matrix organic EL display unit for making display with dots which are arranged in the form of a matrix is of a simple matrix system or an active matrix system.
In the simple matrix system, only organic EL elements form a display panel of a display unit. This system is adapted to directly drive the organic EL elements of respective pixels which are arranged on the display panel in the form of a matrix from the exterior in synchronization with a scanning signal. If the number of scan lines increases, therefore, the driving time (duty) allotted to each pixel is reduced to lower the contrast.
In the active matrix system, on the other hand, a pixel driving element (active element) is provided for each of pixels which are arranged in the form of a matrix. The pixel driving element serves as a switch whose ON and OFF states are switched by a scanning signal. A data signal (display signal) is transmitted to an anode of each organic EL element through each pixel driving element which is in an ON state, thereby driving the organic EL element. When the pixel driving element thereafter enters an OFF state, the data signal applied to the anode of the organic EL element is stored in a storage capacitor in the state of charges, so that the storage capacitor continuously drives the organic EL element until the pixel driving element enters an ON state again. Even if the number of scan lines increases to reduce the driving time allotted to each pixel, therefore, driving of the organic EL element is not influenced and the contrast is not lowered. Thus, the active matrix system can make display with extremely higher picture quality as compared with the simple matrix system.
The active matrix system is roughly classified into a transistor type (three-terminal type) and a diode type (two-terminal type), depending on the types of the pixel driving elements. While the transistor type is hard to fabricate as compared with the diode type, the contrast or resolution can be readily increased. Thus, the transistor type can implement a high-definition organic EL display unit matching the CRT. The above description of the operation principle of the active matrix system mainly corresponds to the transistor type.
Japanese Patent Laying-Open No. 4-125683 (1992) proposes an active matrix system organic EL display unit employing a thin-film transistor (TFT) for each pixel driving element. The thin-film transistor employs a polycrystalline or amorphous silicon film as an active layer. In the organic EL display unit disclosed in the aforementioned gazette, each organic EL element and the TFT are arranged in line on a glass substrate. The TFT has a MOS structure.
The organic EL element requires a low driving voltage and a high driving current. In case of applying a transistor to each pixel driving element of an active matrix system organic EL display unit, therefore, the transistor must be capable of high-current driving. A MOS transistor has higher ON-state resistance as compared with a bipolar transistor. Such high ON-state resistance is disadvantageous for high-current driving. In case of a general bulk MOS transistor having a planar structure, the gate width must be extremely increased and the element area must be increased in order to reduce the ON-state resistance between the source and the drain. In a MOS structure TFT employing a polycrystalline or amorphous silicon film as an active layer, the ON-state resistance between the source and the drain exceeds that of the bulk MOS transistor having a planar structure, and hence the element area must be extremely increased for enabling high-current driving.
In the organic EL display unit disclosed in Japanese Patent Laying-Open No. 4-125683 formed by arranging each organic EL element and each TFT in line on the glass substrate, the element area of the TFT is so limited that it is difficult to sufficiently drive the organic EL element with a high current. When the driving current for the organic EL element is insufficient, it is impossible to obtain an organic EL display unit of high picture quality.
Further, a number of fabrication steps are required for obtaining the MOS structure TFT employing a polycrystalline or amorphous silicon film as an active layer. In addition, the polycrystalline or amorphous silicon film cannot be fabricated in the same process as an organic compound layer forming the organic EL element. Consequently, the organic EL display unit disclosed in Japanese Patent Laying-Open No. 4-125683 disadvantageously requires a high fabrication cost.